A. Technical Field
This invention relates to machining tools. In particular, the invention relates to a precision tool holder designed to enable the machining of precision meso-scale components with complex three-dimensional shapes with sub-micron accuracy on a four-axis lathe.
B. Description of the Related Art
Cutting parts on a lathe is one of the oldest and most versatile machining methods. Modern versions of the lathe include four-axis diamond turning machines (hereinafter “four-axis lathe”), which incorporate a rotary table or platform that allows the cutting tool to swivel with respect to the workpiece to enable the precise machining of complex workpiece forms. The four axes of such a four-axis lathe are illustrated in FIG. 1. The spindle 10, referred to as the “C-axis” of the lathe is typically mounted on a headstock 11 which is translatable in the “x-axis,” so that the spindle and the workpiece 12 may be moved toward or away from an operator. The rotary table 13 is typically mounted on a carriage 14 which is translatable in the “z-axis,” so that a cutting tool 15 mounted on a tool holder 16 mounted on the rotary table may be moved toward or away from the workpiece in a direction parallel to the C-axis of the spindle. The rotary table has a rotational axis referred to as the “B-axis” for rotating the tool in the xz-plane to swivel the tool with respect to the workpiece. The rotary table and B-axis is used when machining parts with complex shapes, such as spheres or oddly shaped lenses, and it is a crucial element for machining parts with both interior and exterior features, such as components for a double shell target.
The accuracy of a machined workpiece surface, however, is generally limited by the accuracy with which the tool is set in place. And in the case of four-axis lathes, the cutting tool must be positioned at the correct height and in the correct lateral position in order to machine parts and components with sub-micron accuracy. In particular, the center of the nose of the single point diamond tool must be at the same height as the C-axis of the spindle and precisely aligned with the B-axis of the rotary table.
Various types of tool holders are commercially available and/or known in the art which enable adjustments for correctly positioning a cutting tool on a four-axis lathe. Generally, tool holders are mounted directly on the rotary table and operate to maneuver the single point tool relative to the rotary table so as to center the nose of the tool along the B-axis. To describe relative motion of the tool with respect to the rotary table, a coordinate system is defined that is fixed to the rotary table, shown in FIG. 1 as the ijk coordinate system. The ijk coordinate system has its origin directly on the B-axis of the rotary table, with the i-axis and j-axis parallel to the rotary table, and the k-axis parallel to the B-axis. The k direction is often referred to as the “height” direction. In this coordinate system, the tool holder maneuvers the tool in the i and j directions to center the tool nose with the B-axis, and also in the k direction to align the height of the cutting edge of the tool with the C-axis of the spindle.
One exemplary type of non-commercial precision tool holder known in the art was custom developed at the Lawrence Livermore National Laboratory specifically for a precision diamond turning machine called, DTM2, which allows the cutting tool to be oriented along the B-axis with nanometer precision, and enables it to be used for machining meso-scale components with complex three-dimensional shapes. This tool holder manipulates the tool in the i and j directions using pre-loaded crossed-roller bearing stages for coarse motion, and piezo stacks for fine motion. The piezo actuators provide only a few microns of motion but can manipulate the tool with a precision on the order of tens of nanometers. The tool is moved in the k direction by a hydraulic lift that provides nanometer positioning precision. This tool holder, however, is a complicated design that requires a control system to drive the piezo actuators and is considered too large in size for modern commercial diamond turning machines.
One exemplary type of commercial tool holder known in the art uses sliding plates that are preloaded with set screws and adjusted with differential screws for making adjustments (within a few microns) in the i, j, and k directions of the ijk coordinate system. Such sliding-plate tool holders, however, can exhibit cross-talk, i.e. where tool adjustment in one of the i and j directions can cause movement by more than 1 μm in each of the other orthogonal directions, which complicates setting the tool with sub-μm accuracy in all the i, j, and k directions simultaneously. For example, depending on the circumstances and the required level of accuracy, setting the tool correctly can take a skilled operator up to 16 hours. In addition, each time the tool is adjusted friction becomes trapped in the sliding components of the tool holder. If proper measures are not exercised this trapped friction can be released in the form of incremental slipping of the sliding plates. This slipping can cause the tool to move several μm over the course of a few days, which further complicates the issue of keeping the tool centered correctly on the B-axis. Each time the tool moves, its position must be reset. Therefore, when machining precision components, a significant portion of the machinist's time is typically spent setting, resetting, and checking the tool position. As such, such sliding-plate tool holders have limited accuracy and cannot position the tool with the precision required for machining complex high-precision parts and components, such as for example high-precision meso-scale laser targets.
It is notable that even if one of the ijk directions (such as the k height direction) did not involve sliding plate actuation, but was rather based on a flexure-based actuator mechanism, the set of sliding plates used for adjustment in the i and j directions would make the adjustment precision in those sliding directions on the order of several microns, which is not suitable for precision machining of complex meso-scale components with sub-μm profile requirements.
It is further notable that many industrial diamond turned parts and components machined on four-axis machine tools are either relatively large or have shallow profiles, such as lenses or molds. For such large workpieces, various metrologies of the workpiece and error-compensation methodologies are available such that high-precision tool holders are not necessarily required, and tool setting errors of several microns or more are tolerable when used in conjunction with such methods. For example, an interferometer or displacement probe, such as a linear variable displacement transducer (LVDT), can be used to measure the profile of the machined part to map the error of the surface figure. The machine tool can then compensate for any workpiece profile error to machine the desired profile. When used in such manner, such commercial tool holders provide a simple means of positioning the tool within a few microns of the axis of rotation of the B-axis, and as such work well for their intended method of operation. However, in contrast to workpieces that are large enough and stable enough to be measured accurately with the LVDT, such error compensation techniques are difficult to apply to meso-scale components with complex three-dimensional shapes, such as components for laser targets, since these components often contain both interior and exterior features or they are too complex or too fragile to be reliably measured. Therefore, in order to correctly machine the sub-micron profiles of such complex meso-scale components, the cutting edge of the tool must traverse the correct path along the workpiece surface which necessitates that the tool be accurately positioned.
Another alternative to precision tool setting on the ijk coordinate system that is known in the art is to establish a virtual axis in the k direction that passes through the actual location of the center of the tool nose. In this method, the machine tool may be programmed in this virtual, tool-centered coordinate system by using combined motion of the x-, z-, and B-axis to produce the desired motion of the tool relative to the center of the tool nose. Such an approach would require only that the actual location of the tool be known, but it would not require that the tool be positioned in any particular location. The advantage of programming in a virtual coordinate system is that commercially available tool holders could be used to position the tool adequately. However, many commercially available four-axis diamond turning machines do not currently have the capability to allow programming in a virtual, tool-centered coordinate system.
What is needed therefore is a tool holder designed to precisely position a single point diamond tool on the B-axis of a four-axis lathe, and capable of adjusting the cutting tool position in three orthogonal directions over a suitable range of motion with sub-micron precision, e.g. up to tens of nanometers precision. Furthermore, such a tool would preferably be immune to any potential detrimental effects of trapped friction, and have very little parasitic error motion, e.g. less than 0.1 micron, to improve the precision and setup efficiency with which complex workpieces, such as meso-scale components, can be fabricated using commercially available four-axis lathes.